This invention relates to an apparatus for driving a piezoelectric actuator including a piezoelectric element used as an electro-mechanical transducer, and more particularly to an apparatus of the kind described above in which the piezoelectric element is suitably charged and discharged so that the piezoelectric element can make a predetermined amount of mechanical displacement.
Various kinds of actuators utilizing a piezoelectric element have been proposed hitherto. This is because the piezoelectric element has various marked advantages including a small size, low power consumption and high speed responsiveness. In this connection, various kinds of circuits for driving such a piezoelectric actuator have also been proposed hitherto. For example, JP-A-62-210241 (the publication of one of the priority applications for U.S. Pat. No. 4,749,897) describes that, in view of the fact that a piezoelectric element has a large temperature coefficient, the mechanical displacement of the piezoelectric element can be controlled to be maintained constant independently of the temperature when the amount of electrostatic energy stored in the piezoelectric element is maintained constant. According to the disclosure of JP-A-62-210241 which intends to control the mechanical displacement of the piezoelectric element to the desired target value, the amount of energy stored in a transformer is controlled so as to control the mechanical displacement of the piezoelectric element on the basis of the energy stored in the transformer. Also, JP-A-58-64077 discloses an apparatus in which a coil is connected to an electrostrictive element (a piezoelectric element) to form a resonance circuit, and electric power stored once in the piezoelectric element is returned by the function of this resonance circuit toward a power source, thereby minimizing the power consumption.
However, in the case of JP-A-62-210241 cited above, in which the energy stored in the transformer is used to directly charge the piezoelectric element thereby controlling the mechanical displacement of the piezoelectric element, the transformer is required to be very large in size so that it can store a large amount of energy when a high speed response and high energy are demanded for controlling the displacement of the piezoelectric element.
FIG. 5 is a graph showing the voltage-displacement characteristic of such a piezoelectric actuator. In order that the piezoelectric actuator can make maximum expansion and contraction, it is preferable that the piezoelectric element operates at a voltage between points A and B in FIG. 5 relative to the hysteresis curve of the piezoelectric element.
Also, in the case of JP-A-58-64077 cited above, in which the resonance circuit is used to minimize the power consumption, a negative voltage is induced in the piezoelectric element when the electric charge stored once in the piezoelectric element is returned toward the power source by the resonance circuit, and the value of this negative voltage is determined by both the value of a positive voltage applied to the piezoelectric element and the constants of the resonance circuit. Thus, a change in the value of the positive voltage applied to the piezoelectric element results in a great change in the value of the negative voltage induced in the piezoelectric element. Therefore, the hysteresis curve of the piezoelectric element will differ in each operation cycle, with the result that the displacement of the piezoelectric element may not be controlled to the desired target value or may become unstable. Also, there arises such a problem that undesirable deterioration of the piezoelectric element will be accelerated when a negative voltage higher than the voltage at the point B in FIG. 5 is applied to the piezoelectric element.